calculating traveller/sheet loads for square sails

I'm not at all sure on what this formula is based upon; it appears to be something specific to Harken though it is reprinted in Brion Toss's 'Apprentice.

The constant used here is likewise not explained, but it is modified upward for metric measures and so is likely related to surface area. My guess is that it approximates developed force at sea level/high humidity of air.

::doodles with numbers for Cape Dory 25D::

ML = (105.0625 * 742.5625 * 0.00431 * 100) / (29.1140000687 * 8.75)
ML = 131.99214373252

This seems rather excessive for 10 knots of wind, for one thing.

I don't think this is a reasonable formula to determine the kind of load might develop on your mizzen. A simpler method to get an estimate would be to multiply maximum sail area carried by an estimated dynamic pressure (q) for the maximum wind that much sail would be carried in, times the height of the center of effort of the sail above the step.

To figure out the dynamic pressure you can work out the formula:

q = (p / 2) * V^2

where q is the dynamic pressure, p = 0.002378 slugs per ft^3 (standard sea level conditions estimate), and V = wind speed in ft per second.

Or you could check this quick ugly table:

[code]
V in Beaufort q
knots number lbs per ft^2
----- ------- -----------
10 3 0.33
20 5 1.3
30 7 3
40 8 5
50 10 8
60 11 12
70 12 17
80 13 22
90 15 28
100 16 34[/code]
(Earl R. Hinz "The Complete Book of Anchoring and Mooring" 2d edition)